1. Field of the Invention
This invention relates to a radiation image read-out method for exposing a stimulable phosphor sheet, on which a radiation image having a background region has been stored, to stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, and detecting the emitted light in order to obtain an image signal representing the radiation image.
2. Description of the Related Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored during exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used to reproduce the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT), or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light emitted by the stimulable phosphor varies over a wide range and is proportional to the amount of energy stored during its exposure to radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order to obtain a desirable image density, an appropriate read out gain is set when the emitted light is being detected with a photoelectric read-out means and converted into an electric signal to be used in the reproduction of a visible image on a recording material, such as photographic film, or a display device such as a CRT.
FIG. 3 is a schematic view showing an example of the radiation image stored on a stimulable phosphor sheet.
With reference to FIG. 3, a radiation image 2 is stored on a stimulable phosphor sheet 1. In the course of recording a radiation image of an object on a stimulable phosphor sheet 1, an irradiation field stop is often used in order to limit the irradiation field to an area smaller than the overall recording region of the stimulable phosphor sheet 1 so that radiation is irradiated only to that portion of the object, which is to be viewed, and part of the stimulable phosphor sheet 1. When the radiation image 2 is recorded on the stimulable phosphor sheet 1, an irradiation field stop is used so that no radiation will be irradiated to peripheral regions 1a, 1a of the stimulable phosphor sheet 1, and the radiation image 2 will be recorded only in a middle region 1b of the stimulable phosphor sheet 1. The radiation image 2 is composed of an object image 2a recorded with radiation, which has passed through the object, and a background region 2b upon which radiation impinges directly without passing through the object. In general, when the radiation image 2 is read out from the stimulable phosphor sheet 1 and an image signal representing the radiation image 2 is obtained, the stimulable phosphor sheet 1 is conveyed in the subscanning direction indicated by the arrow Y, and at the same time a spot of stimulating rays repeatedly scans the stimulable phosphor sheet 1 in the main scanning direction indicated by the arrow X. As a result, the whole area of the stimulable phosphor sheet 1 is scanned with the spot of stimulating rays. Light emitted from every position on the stimulable phosphor sheet 1, which is being scanned with the spot of stimulating rays, is photoelectrically detected and converted into a image signal.
Problems occurring when the spot of stimulating rays scans positions on the stimulable phosphor sheet 1, which lie along a main scanning line .xi., will be described hereinbelow with reference to FIG. 3.
The spot of stimulating rays scans the stimulable phosphor sheet 1 rightwardly along the main scanning line .xi.. When a position 3 in the object image 2a is being scanned with the spot of stimulating rays after the background region 2b has been scanned, an amount of light is emitted from the position 3, which is proportional to the intensity of the stimulating rays and to the amount of energy stored at the position 3 during its exposure to radiation.
However, when a certain position on the stimulable phosphor sheet 1 has already been scanned and the spot of stimulating rays is now being irradiated to a next position, an after-glow continues to emanate for a while from the position which has already been scanned with the spot of stimulating rays. In the background region 2b, a very large amount of energy is stored during exposure of the stimulable phosphor sheet 1 to radiation. Therefore, a very high intensity of light is emitted by the background region 2b when it is exposed to stimulating rays, and a very large amount of after-glow will emanate therefrom. Accordingly, when stimulating rays are being irradiated to the position 3 shown in FIG. 3, an after-glow will be emanating from positions 5 in the background region 2b which have already been exposed to stimulating rays. The after-glow will impinge upon the photodetector together with the light emitted from the position 3 which is being scanned. As a result, even if a small amount of energy is stored at the position 3 which is being scanned (and therefore a low image density should be reproduced at the corresponding position in the visible image), an image signal representing a large amount of stored energy (i.e. representing a high image density) will be obtained from the position 3 because of the after-glow emanating from the background region 2b. When a visible image is reproduced from an image signal thus obtained, the image density of that part of the reproduced visible image corresponding to the part of the object image 2a adjacent the background region 2b becomes high along a line corresponding to the main scanning line .xi.. (Such a phenomenon is referred to as the tailing phenomenon.) Consequently, the image quality of the reproduced visible image becomes poor.
Also, in cases where stimulating rays include flare, the flare will impinge upon positions 4 around the position 3 which is being scanned with stimulating rays. The flare causes the positions 4 to emit light. The light emitted from the positions 4 will impinge upon the photodetector together with the light emitted from the position 3 which is being scanned. Therefore, the flare also causes the image quality of the reproduced visible image to become poor. When such flare impinges upon the background region 2b, particularly large adverse effects will occur. As described above, because radiation directly impinges upon the background region 2b without passing through the object when a radiation image 2 is recorded on the stimulable phosphor sheet 1, a very large amount of energy will be stored in the background region 2b. Therefore, even if the amount of flare is very small, a considerable amount of light will be emitted from the background region when it is exposed to the flare. Specifically, even if a small amount of energy is stored at the position 3 which is being scanned (and therefore a low image density should be reproduced at the corresponding position of the visible image), an image signal representing a large amount of stored energy (i.e. representing a high image density) will be obtained from the position 3 because of the light emitted from the background region 2b during its exposure to the flare. Therefore, when a visible image is reproduced from an image signal thus obtained, the tailing phenomenon occurs in the reproduced visible image, and the image quality of the reproduced visible image becomes poor.